Will try to explain:

At the end of previous season I bought a GK. He was 5* and needed 1% to become a 6*.
After I won the auction he was still 5* but suddenly needed 20% to become a 6*.
This afternoon my son, who plays too, had exact the same problem, only not with a GK.
What the hell is this. Ofcourse I dont have screenshot, I mean, who expect this to happen.